Sound level (decibel) meters

Have you even been in a noisy factory and had to cover your ears? Walked past a jackhammer in the street and winced because the
sound
was so loud? Being exposed to loud noises for a brief period of time
usually does no harm, but imagine having to suffer it
hour upon hour, day after day. If noise is a problem to you, the first thing you
need to do is measure how loud it is so you can take effective steps to reduce it.
Making precise measurements of noise used to be quite a tricky business, but now
there are automated, electronicsound-level meters that do the job
for you. Let's take a closer look!

Photo: A typical sound pressure level (SPL) meter made by Quest.
Note the long stick at the top: it's the microphone that
samples and measures incoming sounds. Photo by James R. Evans courtesy of
US Navy.

What makes one sound louder than another?

How loud a sound seems to be depends on who's listening. A young
person playing rock up in their bedroom might not think their music
is loud, but their parents in the room down below might have other
ideas. In other words, how loud things seem is a subjective
thing and not something we can easily measure. However, what makes
one sound seem louder than another is the amount of energy that the
source of the sound is pumping towards the listener in the form
of pressure variations in the air. That's the intensity of the
sound and it's an objective thing—something we can easily measure
and agree on.

Meters that measure sound levels work by calculating the pressure of the sound
waves traveling through the air from a source of noise. That's why
you'll sometimes see them referred to as sound pressure level (SPL)
meters. Devices like this give a measurement of sound intensity in
units called decibels, a scale first devised by
telephone pioneer Alexander Graham Bell (decibel~Bell, get it?).

Understanding the decibel scale

You have to think about the decibel scale very carefully, because it's a
logarithmic scale and it works in a different way to the scale on a ruler, which is a linear scale. On a ruler, a distance of 20cm is twice as long as a
distance of 10cm and 30cm is three times as long. But the logarithmic decibel
scale goes up in powers of ten: every increase of 10dB on
the scale is equivalent to a 10-fold increase in sound intensity (which broadly corresponds with loudness).
That means a sound of 20dB is 10 times louder than a sound of 10dB and a 30dB sound is
100 times louder. A sound of 100dB is actually
1,000,000,000 times louder than a sound of 10dB and not 10 times as
loud, as you might suppose. That's why sounds high up the decibel scale (from about 85-200dB) are
a major cause for concern: the sound waves carry so much energy that they will damage your hearing, sooner or later.

Photo: Imagine how loud this job can get. The Landing Signal Enlisted (LSE) is the brave person who has to guide helicopters and jet airplanes safely in to land on aircraft carriers. Notice that they always wear heavy duty earmuffs to protect their hearing against jet engine noise that can exceed 150dB at such close quarters. Photo by Patrick Grieco courtesy of US Navy.

The Decibel Scale

From rustling leaves to jet engines, here's a quick guide to everyday loudness...

Level in decibels

Typical everyday example

Times louder than 10dB

10dB

Rustling or falling leaves.

1

20dB

Watch ticking.

10

30dB

Birds flying by.

100

40dB

Quiet conversation.

1,000

50dB

Louder conversation.

10,000

60dB

Quiet traffic noise.

100,000

70dB+

Louder traffic

1,000,000

80dB+

Loud highway noise at close range

10,000,000

85dB

Hearing damage after approximately 8 hours.

100dB

Jackhammer (pneumatic drill) at close range

1,000,000,000

100dB

Hearing damage after about 15 minutes.

110dB+

Jet engine at about 100m

10,000,000,000

120dB

Threshold of pain. Hearing damage after very brief exposure.

How sound level meters work

Sound level meters look quite simple. They have a pointy stick at the top, which
is the microphone that samples and measures the sound. The stick
keeps the microphone away from the body of the instrument, cutting
out reflections, and giving a more accurate measurement. Inside the
square box at the bottom of the meter, electronic circuits measure
the sound detected by the microphone and amplify and filter it in
various ways before showing a readout on a digital LCD display.

Different kinds of sound level meters

Different kinds of sound level meters are available. The most basic ones give a
reading of the maximum instantaneous sound pressure level (SPL), in
decibels, at the moment when you switch them on. Roughly speaking,
that corresponds to how loud a noise sounds at the moment you take
the measurement. That isn't always tremendously useful if you want to measure the average sound levels
over a period of time in somewhere like a noisy production plant. For
that, you need a slightly more sophisticated (and expensive) device
called an integrating sound level meter. Integrating, in this case,
means summing over a period of time. For measuring sound
levels over longer periods, you can use data logging meters that make
measurements every so often. Upload these measurements to a computer
and you can draw yourself a chart showing how the sound level
varies over the course of a day, week, or longer. This kind of
longer-term analysis is a fairer and more accurate way of sensing
whether sound is a nuisance or a health issue than simply relying on
one or two instantaneous measurements.

Photo: A close-up of the LCD display in the Quest Model-2900 sound level meter
in our top photo, currently reading 84.7 decibels (just below the level considered to cause long-term hearing damage). This sophisticated meter is capable of integrating and logging as well as making instantaneous measurements.
It's an example of a type-2 meter (see below).
Photo by James R. Evans courtesy of US Navy.

Types of sound level meters

Sound level meters also vary widely in quality. The best ones (indeed, the only
ones worth using) are designed to meet international standards
such as IEC 60651, IEC 60804 and ANSI S1.4 and are graded as
type (or class) 0–3. Meters that meet the highest standards are called type 0
and they're suitable for making high-precision
measurements (in a scientific laboratory, for example).
Type 1 are slightly less accurate, but still suitable for high-precision work.
Those that meet the lesser standards are called type 2 meters
and are suitable for more general use, but not really for lab-quality measurements.
Type 3 meters, which are considerably less expensive, are less accurate again and only really suitable for rough survey work or for making preliminary measurements to see whether more accurate
surveys are needed with a type 0-2 meter.

Virtual sound level meters

You have a microphone and you have a laptop, so why not combine the two to make a kind of virtual software meter?
You'll need to calibrate your equipment (match sound levels of known loudness to the traces on your screen so you
can find the levels of unknown sounds). Then you can use your laptop in exactly the same way as any other sound
level meter. You can also find some fairly good sound level apps for things like the iPhone, which do an even better job.
While you can't really expect DIY meters and phone apps to match the quality or accuracy of the very best professional (type 0/1)
sound meters, which cost 10–100 times more, they can certainly give you a reasonably good indication of the relative loudness of different sounds.

Here are a few examples of sound-level software you can download and experiment with:

SPL Meter: A "professional-grade sound level meter" that's part of AudioTools, an app designed for the iPhone and iPod Touch.

Audacity: A general-purpose sound-editing program that you can also use for crude sound measurements. It's not really suitable for accurate measurements, however.